Gas-blast circuit breakers

ABSTRACT

A gas-blast circuit breaker comprising, within a switching chamber permanently filled with extinguishing gas under overpressure, a first and a second switching contact, the switching-off action of said switching contacts being effected by pistons which are spring biased against such action and are mounted within cylinders associated with said switching contacts, respectively, at least one of said cylinders being permanently in open communication with said switching chamber, a locking member being provided which is adapted to be released by a switching-off command and which is holding the switching contacts in their respective rest positions corresponding to the closed condition of the circuit breaker.

United States Patent Frowein [4 1 Mar. 14, 1972 [54] GAS-BLAST CIRCUIT BREAKERS [72] inventor: Egbertus A. Frowein, Arnhem, Netherlands [73] Assignee: N.V. COQ, Utrecht, Netherlands [22] Filed: Dec. 10, 1969 [211 App]. No.: 883,808

[30] Foreign Application Priority Data Dec. 20, 1968 Netherlands ..6818386 [52] US. Cl. ..200/l48 R [51] Int. Cl. H011! 33/86 [58] Field oiSearch....; ..200/l48, 148 B, 148 D, 148 G, 200/150 B [56] References Cited UNITED STATES PATENTS 2,457,939 1/1949 Stroup ..200/150 B 2,949,521 8/1960 Schulz ..200/ 148 B 3,339,046 8/1967 Giammona et a1. ..200/ 148 B FOREIGN PATENTS OR APPLICATIONS 347,562 8/1960 Switzerland ..200/148 B Primary Examiner-r-Robert S. Macon Attorney-Snyder and Butrum [s71 ABSTRACT A gas-blast circuit breaker comprising, within a switching chamber permanently filled with extinguishing gas under overpressure, a first and a second switching contact, the switchingoff action of said switching contacts being efi'ected by pistons which are spring biased against such action and are mounted within cylinders associated with said switching contacts, respectively, at least one of said cylinders being permanently in open communication with said switching chamber, a locking member being provided which is adapted to be released by a switching-off command and which is holding the switching contacts in their respective rest positions corresponding to the closed condition of the circuit breaker.

9 Claims, 1 Drawing Figure PATENTEDMAR 14 I972 3, 649 792 INVENTOR [:qasn rus/l- E ATTORNEY GAS-BLAST CIRCUIT BREAKERS The invention relates to a gas-blast circuit-breaker comprising a switching-chamber which is permanently filled with extinguishing gas under overpressure and in which a first switching contact and a second switching contact cooperating therewith are provided, said switching contacts, of which at least one is formed as a nozzle contact acting as discharge passage for the extinguishing gas, being both mounted for axial movement and each being coupled with a piston mounted for reciprocation in a cylinder and controlled by the extinguishing gas, each piston being loaded by a spring counteracting the switching-off movement, an annular valve surrounding the switching contacts, said valve being closed and separating the switching chamber from the switching contacts, when the circuit-breaker is in its closed state of rest, and controlling members which are adapted to be set in action by a switching-off command and make that first both switching contacts are driven and accelerated together in the switchingoff direction by the pressure of the extinguishing gas acting only on one sideof the first piston which is coupled with said first switching contact, and said valve is opened, so that one side of the second piston, which is coupled with said second switching contact, is exposed to the pressure of the extinguishing gas and the second switching contact is also driven in the switching-off direction by its own piston and continues to move together with the first switching contact, thereupon the movement of the first switching contact is checked by a stopmember, so that only the second switching contact moves-on and the switching contacts are separated, and finally at the end of a predetermined time interval after said switching-off command the gas pressures on both sides of both pistons are brought to such values, that the switching contacts are forced back to their positions of rest at least by the springs acting on them and the circuitbreaker is returned to its closed state.

A gas-blast circuit-breaker of this construction is disclosed in the Dutch Pat. application No. 67.05277 laid open to public inspection. If sulphur hexafluoride (SP gas is used as extinguishing gas, one has to reckon with the fact, that the velocity of sound in that gas is low, so that filling cylinder spaces with said gas takes a rather long time and, in order to obtain a quick reaction of the circuit-breaker after the receipt of a switchingoff command, wide conduits and ports are required. The invention has the object to provide a gas-blast circuit-breaker which, also when SF, gas is used as extinguishing gas, reacts quickly on the switching-off command without the neccesity of using passages of unpractical wide dimensions, said circuitbreaker having an additional advantage which will be described herein after. The object aimed at is obtained by the invention, in that the space of the cylinder, containing the first piston, intended for the switching-off movement is and remains in open communication with the switching chamber and the spaces of both cylinders, intended for the reclosing movement are in open communication with each other and are adapted to be connected by a common changeover valve either with the switching chamber or with the atmosphere of a space containing extinguishing gas, the pressure of which is lower than that of the gas contained in the switching chamber, whereas a locking member adapted to be released by the switching-off command is provided, said locking member holding the switching contact mechanically in their positions of rest corresponding to the closed condition of the circuitbreaker. Consequently, the cylinder space intended for starting the switching-off movement is permanently filled with extinguishing gas having the high pressure of the gas contained in the switching chamber, so that after releasing the locking member the switching contacts are immediately driven in the switching-off direction. Due to this fact the time which has lapsed between the receipt of the switching-off command and the separation of the switching contacts can be reduced to less than one-third of the usual time required therefor.

An other advantage of the fact that the said cylinder space is permanently kept under high pressure is that also in the closed condition of rest of the circuit-breaker the switching contacts are pressed with their end faces which are provided with areing material against each other with great force and remain so till they are separated. Due there to the repeated impingement of said end faces against each other during the first stage of the switching-off process and the possibility of damaging the mostly brittle arcing material are avoided and the current will flow through said end faces operating as arcing surfaces also before the switching contacts are separated. This results in that the transition or the commutation of the current from the individual fixed main contacts for the permanent current conduction to the arcing parts of the switching contacts takes place without additional phenoma, so that it is optimally favorable.

In order to obtain a quick reaction of the locking member on the switching-off command it is advantageous to construct the circuit-breaker in such a way, that the locking member is forced into the locking position by a spring and is held in the locking position by a second locking member adapted to be released by the switching-off command said first locking member being coupled with a piston mounted for movement in a double-acting cylinder, of which the space intended for the releasing action is and remains in open communication with the switching chamber and the space intended for the locking action is and remains in open communication with the cylinder space intended for reclosing the second switching contact. The second locking member can be a very light member, whereas also in this case the cylinder space for setting a member, that means the first locking member, in action is permanently filled with gas of high pressure, so that said cylinder space need not be filled up to the high gas pressure after the switching-off command has been received.

The time interval between the separation and the reclosing of the switching contacts, said time interval being necessary to extinguish the switching arc, can be obtained in a favorable manner, when the circuit-breaker is constructed such, that the changeover valve is controlled by a spring and a piston mounted for movement in a cylinder, of which the active space is in open communication with a chamber via a conduit having a delaying action, and a second changeover valve is provided which is coupled with the first locking member and adapted to connect said chamber either with the switching chamber or with the atmosphere or a space containing extinguishing gas, the pressure of which is lower than that of the gas contained in the switching chamber.

The invention will be elucidated with the aid of the drawing which shows an axial sectional view of the switching element of a gas-blast circuit-breaker.

FIGS. 1 and 2 show contact holders which are provided with electrically conductive hollow blocks 3, 4a, 4b kept at a distance from each other by a tubular insulator 5.

The block 3 is provided with a double-acting cylinder 6, in which a piston 7 is mounted for reciprocation. Attached to said piston is a first axially movable tubular switching contact 8 which also acts as discharge passage 9 for the extinguishing gas. The piston 7 is loaded by a spring 10.

The block 4a is provided with a cylinder 11, in which a plunger piston 12 is mounted for movement. Attached to the plunger piston 12 is a second axially movable tubular switching contact 13 which also forms a discharge passage 14 for the extinguishing gas. The plunger piston 12 is loaded by a spring 15. The switching contacts 8, 13 are surrounded by an annular valve, which is constituted by a seat ring 16 connected by a resilient bellows 17 with the cylinder 6 and by an annular valve body 18 which is attached to the plunger piston 12. In the illustrated closed condition of rest of the circuit-breaker the valve 16, 18 is closed and the switching contacts 8, 13 and their discharge passages 9, 14 for the extinguishing gas are separated from the switching chamber 19 contained within the blocks 3, 4a and the insulator 5 and surrounding the cylinders 6, 11 said switching chamber 19 containing extinguishing gas under high overpressure and being in open communication with an extinguishing gas supply system (not shown) through wide openings 20,21.

ln the shown closed condition of the circuit-breaker the second switching contact 13 is electrically conductively connected through resilient contact rings 22 with the block 4b and the contact holder 2 and through a circular series of contact fingers 23 with the cylinder 6, the block 3 and the contact holder 1. The first switching contact 8 is electrically conductively connected through a resilient contact ring 24 with the cylinder 6, the block 3 and the contact holder 1.

The cylinder space 6a intended for the switching-off operation is in permanent open communication with the switching chamber 19 via wide openings 25, whereas the cylinder space 6b intended for the reclosing operation is permanently connected with a chamber 27 through an opening 26. This chamber can be brought into communication by a changeover valve 28 either through ports 29 with the atmosphere or a space 30 filled with low-pressure extinguishing gas or through ports 31 with the switching chamber 19 filled with high-pressure extinguishing gas. The chamber 27 is in open communication with the cylinder space 11b intended for the reclosing of the second switching contact by means of a wide tube 32 of insulating material and a block having a passage 33. The valve 28 is coupled by a rod 34 with a piston 37 mounted for movement in a cylinder 35 and loaded by a spring 36.

The second switching contact 13 cooperates with a first locking member 38 having the shape of a cam formed in a transverse boring 39 of a piston 40, said cam engaging in the closed condition of the circuit-breaker an annular groove 41 of the switching contact 13 extending through said transverse boring. Said cam locks the switching contacts 8, 13 mechanically in their closed positions. The piston 40 is mounted for reciprocation in a double-acting cylinder 42, of which the cylinders space 42a intended for the releasing operation is in open communication with the switching chamber 19 through the opening 43 and the cylinder space 42b intended for the locking operation is in open communication with the cylinder space 1 1b through the opening 44. The piston 40 is loaded by a spring 45 coupled with a second changeover valve 46 by means of a rod 47 provided with a head 48 which cooperates with a second locking member in the form of a pawl 49 which in the closed condition of rest of the circuit-breaker holds the first locking member 38 in its locking position, said pawl being adapted to be retracted by a switching-off command.

The valve 46 connects a chamber 50 either through ports 51 with a low-pressure space 30 or through ports 52 and an opening 53 with the high-pressure switching chamber 19. The chamber 50 is connected by means of a conduit with delaying action in the shape of a choke passage, which is adjusted by a screw 54, and a tube 55 of insulating material with the active space 56 of the cylinder 35 containing the piston 37 to control the first changeover valve 28.

This circuit-breaker operates as follows:

In the illustrated closed condition of rest of the circuitbreaker the switching contacts 8, 13am locked mechanically in their closed positions by the cam 38 provided in the piston 40. In this condition the switching chamber 19, the cylinder space 6a and the cylinder space 420 are filled with high-pressure extinguishing gas, whereas the cylinder spaces 6b, 11b, 42b and 56 are in open communication with the low-pressure space 30. Also all spaces inside the valve l6, 18 such as the chamber 57 and the discharge passages 9, 14 for the extinguishing gas are connected either with the atmosphere or with a low-pressure space. The first switching contact 8 is pressed by the high pressure in the cylinder space 60 with its end face clad with arcing material 58 with great force against the arcing material 59 covering the end face of the second switching contact 13 which is held in its closed position by the locking member 38, so that also a well-conductive path is formed through the arcing surface layers of the switching contacts.

If the circuit-breaker must be opened the pawl 49 is retracted by the switching-off command. Immediately thereafter the piston 40 is driven to the right by the high pressure in the cylinder space 420, so that the locking member 38 is released and the switching contacts 8, 13 are driven and accelerated together in the switching-off direction by the high pressure in the cylinder space 60. As a result of the switchingoff movement of the switching contacts 8, 13 the valve 16, 18 is immediately opened, so that also the plunger piston 12 of the second switching contact 13 is exposed to the pressure of the extinguishing gas contained in the switching chamber 19. The switching contacts 8, 13 continue to move together, so that somewhat later the contact fingers 23 come into contact with the first switching contact 8 and the conduction of the current is then effected only via the arcing end faces of the switching contacts 8, 13 which are still pressed with force against each other. Thereupon the movement of the first switching contact 8 is checked, when the piston 7 strikes against the stop block 60. The switching contacts 8, 13 are then separated, so that the extinguishing gas will flow with force from the switching chamber 19 to the discharged passages 9, l4 and is thereby adapted to extinguish the produced switching are.

As a result of the movement of the piston 40 to the right the valve 46 is also moved to the right, so that the ports 51 are closed and the ports 52 are opened. This has the effect that high-pressure gas flows from the switching chamber 19 via the chamber 50, the choke passage 54 and the tube 55 to the cylinder space 56 which then becomes filled, after a time interval adjusted by the screw 54a, with a sufficient quantity of gas to overcome the pressure of the spring 36 and to changeover the valve 28. This results in that the ports 29 are closed and the ports 31 are opened, so that via the opening 26 the cylinder space 6b, via the chamber 27, the tube 32 and the passage 33 the cylinder space 11b and via the cylinder space 11b and the opening 44 the cylinder space 42b are filled with gas under high pressure. The pistons 7, 12 and 40 are thereby pneumatically discharged, so that they are forced back to their positions of rest by their springs l0, l5 and 45. The switching contacts 8, 13 then return to the closed condition and afier said condition has been attained the locking member 38 is forced into the groove 41 again. Also the valve 56 is changedover again, so that the ports 51 are opened and the ports 52 are closed and due thereto the pressure in the cylinder space 56 falls off. The spring 36 then forces the valve 28 back to its starting position, in which the ports 29 are opened and the ports 31 are closed, so that the cylinder spaces 6b, 11b and 42b are again brought into communication with the low-pressure space 30 and the circuit-breaker is restored to its closed condition of rest. Since the switching-oh command was ended at an earlier moment the first locking member 38 will be held again in the locking position by the pawl 49.

What I claim is:

1. A gas-blast circuit-breaker comprising a switching chamber which is permanently filled with extinguishing gas under high overpressure, a first axially movable switching contact, a second axially movable switching contact cooperating with said first contact, said switching contacts being accommodated in said switching chamber, an axial discharge passage for extinguishing gas from the switching chamber being provided in at least one of said switching contacts, two doubleacting cylinders, two pistons, each piston being mounted for reciprocation in a respective cylinder and coupled with a respective switching contact, the first switching contact and its piston having a shorter stroke, the second switching contact and its piston having a longer stroke, a stationary annular valve member surrounding the first switching contact, a movable annular valve member attached to and surrounding the second switching contact, both valve members forming together a valve which is closed and separates said discharge passage from said switching chamber, when the switching contacts contact each other in the condition of rest of the circuit breaker, and is opened, when the switching contacts are set in motion after the receipt of a switching-off command, springs acting on the switching contacts to force them to their positions of rest, a locking member acting on the second switching contact and adapted to hold mechanically both switching contacts in their positions of rest corresponding to the closed condition of the circuit breaker, a conduit with a changeover valve being connected to both the cylinder space of the first double-acting cylinder to drive the first switching contact and its piston towards their positions of rest and to the cylinder space of the second double-acting cylinder to drive the second switching contact and its piston towards their positions of rest, in which the circuit breaker is in. its closed condition, said changeover valve being adapted to connect both respective cylinder spaces simultaneously either with the switching chamber containing gas under high overpressure or with a space containing gas under less pressure, and keeping said cylinder spaces in communication with said space of less pressure, when the circuit breaker is in its condition of rest and closed, the cylinder space of the first double-acting cylinder for driving the first switching contact and its piston in the switching-off direction being permanently in communication with the high-pressure switching. chamber, the cylinder space of the second double-acting cylinder for driving the second switching contact and its piston in the switching-off direction being in open communication with the space within the annular valve and being separated by the latter from the high-pressure switching chamber, when the circuit breaker is in its closed condition and said valve is closed, means for driving said changeover valve, means for releasing said locking member and operating on a switching-off command and means having a delaying action, said latter means being provided between said means for releasing said locking member and the means for driving said changeover valve into its position, in which the cylinder spaces of both cylinders to drive the switching contacts and their pistons towards their positions of rest are in communication with the high-pressure switching chamber.

2. A gas-blast circuit-breaker according to claim 1, characterized in that the locking member is forced into the locking position by a spring and is held in the locking position by a second locking member adapted to be released by the switching-off command, said first locking member being coupled with a piston mounted for movement in a double-acting cylinder, of which the space intended for the releasing action I is and remains in open communication with the switching chamber and the space intended for the locking action is and remains in open communication with the cylinder space intended for reclosing the second switching contact.

3. Gas-blast circuit-breaker according to claim 1, characterized in that the changeover valve is controlled by a spring and a piston mounted for movement in a cylinder, of which the active space is in open communication with a chamber via a conduit having a delaying action and a second changeover valve is provided which is coupled with the first locking member and adapted to connect said chamber either with the switching chamber or with the atmosphere or space containing extinguishing gas, the pressure of which is lower than that of the gas contained in the switching chamber.

4. in a gas-blast circuit-breaker, in combination:

housing means defining a chamber maintained in communication with a supply of pressurized extinguishing gas;

a pair of axially aligned switching contacts having opposed end portions movable within said chamber between a closed condition and a separated open condition, at least a single contact of said pair being tubular and communicating exteriorly of said chamber with an environment of substantially less pressure than the pressure of said extinguishing gas so that extinguishing gas may escape in a blast from said chamber through said tubular contact when said end portions are separated;

latch means for holding one of said contacts in closed condition;

actuating means for moving both of said contacts in the same axial direction from closed to open condition, said actuating means including first piston-cylinder means associated with said one contact and second piston-cylinder means associated with the other contact, said second piston-cylinder means includingport means communicating with said chamber for continuously exertlng a force on said other contact in said one axial direction whereby forcibly to engage said end portions of the contacts when same are closed and said latch means is holding said one contact;

means for limiting axial movement of said other contact to a stroke less than that of said one contact;

means for releasing said latch means to initiate opening movement of said contacts due to the force continuously exerted by said second piston-cylinder means;

contact movement-responsive valve means for communicating said first piston-cylinder means with said chamber in response to movement of said contacts from closed condition to force said first contact in said one axial direction and move it beyond said other contact to its open condition when said other contact reaches the end of its stroke.

5. in a gas-blast circuit-breaker as defined in claim 4 including a fixed switching contact bridging across said end portions of said pair of contacts and engaging said one contact when said pair of contacts are in closed condition, the stroke of said other contact being sufficient to carry it beyond said fixed contact whereby a circuit-breaking arc is preferentially drawn between said end portions of said pair of contacts.

6. In a gas-blast circuit-breaker as defined in claim 5 including delayed action valve means for balancing the pressurized extinguishing gas across said first and second piston-cylinder means in delayed response to actuation of said means for releasing said latch means; and spring means for returning said pair of contacts to closed condition when said piston-cylinder means are so balanced.

7. In a gas-blast circuit-breaker according to claim 6 wherein said delayed action valve means also causes return of said latch means.

8. In a gas-blast circuit-breaker as defined in claim 4 including delayed action valve means for balancing the pressurized extinguishing gas across said first and second piston-cylinder means in delayed response to actuation of said means for releasing said latch means; and spring means for returning said pair of contacts to closed condition when said piston-cylinder means are so balanced.

9. In a gas-blast circuit-breaker according to claim 8 wherein said delayed action valve means also causes return of said latch means. 

1. A gas-blast circuit-breaker comprising a switching chamber which is permanently filled with extinguishing gas under high overpressure, a first axially movable switching contact, a second axially movable switching contact cooperating with said first contact, said switching contacts being accommodated in said switching chamber, an axial discharge passage for extinguishing gas from the switching chamber being provided in at least one of said switching contacts, two double-acting cylinders, two pistons, each piston being mounted for reciprocation in a respective cylinder and coupled with a respective switching contact, the first switching contact and its piston having a shorter stroke, the second switching contact and its piston having a longer stroke, a stationary annular valve member surrounding the first switching contact, a movable annular valve member attached to and surrounding the second switching contact, both valve members forming together a valve which is closed and separates said discharge passage from said switching chamber, when the switching contacts contact each other in the condition of rest of the circuit breaker, and is opened, when the switching contacts are set in motion after the receipt of a switching-off command, springs acting on the switching contacts to force them to their positions of rest, a locking member acting on the second switching contact and adapted to hold mechanically both switching contacts in their positions of rest corresponding to the closed condition of the circuit breaker, a conduit with a changeover valve being connected to both the cylinder space of the first double-acting cylinder to drive the first switching contact and its piston towards their positions of rest and to the cylinder space of the second double-acting cylinder to drive the second switching contact and its piston towards their positions of rest, in which the circuit breaker is in its closed condition, said changeover valve being adapted to connect both respective cylinder spaces simultaneously either with the switching chamber containing gas under high overpressure or with a space containing gas under less pressure, and keeping said cylinder spaces in communication with said space of less pressure, when the circuit breaker is in its condition of rest and closed, the cylinder space of the first double-acting cylinder for driving the first switching contact and its piston in the switching-off direction being permanently in communication with the high-pressure switching chamber, the cylinder space of the second double-acting cylinder for driving the second switching contact and its piston in the switching-off direction being in open communication with the space within the annular valve and being separated by the latter from the high-pressure switching chamber, when the circuit breaker is in its closed condition and said valve is closed, means for driVing said changeover valve, means for releasing said locking member and operating on a switching-off command and means having a delaying action, said latter means being provided between said means for releasing said locking member and the means for driving said changeover valve into its position, in which the cylinder spaces of both cylinders to drive the switching contacts and their pistons towards their positions of rest are in communication with the high-pressure switching chamber.
 2. A gas-blast circuit-breaker according to claim 1, characterized in that the locking member is forced into the locking position by a spring and is held in the locking position by a second locking member adapted to be released by the switching-off command, said first locking member being coupled with a piston mounted for movement in a double-acting cylinder, of which the space intended for the releasing action is and remains in open communication with the switching chamber and the space intended for the locking action is and remains in open communication with the cylinder space intended for reclosing the second switching contact.
 3. Gas-blast circuit-breaker according to claim 1, characterized in that the changeover valve is controlled by a spring and a piston mounted for movement in a cylinder, of which the active space is in open communication with a chamber via a conduit having a delaying action and a second changeover valve is provided which is coupled with the first locking member and adapted to connect said chamber either with the switching chamber or with the atmosphere or space containing extinguishing gas, the pressure of which is lower than that of the gas contained in the switching chamber.
 4. In a gas-blast circuit-breaker, in combination: housing means defining a chamber maintained in communication with a supply of pressurized extinguishing gas; a pair of axially aligned switching contacts having opposed end portions movable within said chamber between a closed condition and a separated open condition, at least a single contact of said pair being tubular and communicating exteriorly of said chamber with an environment of substantially less pressure than the pressure of said extinguishing gas so that extinguishing gas may escape in a blast from said chamber through said tubular contact when said end portions are separated; latch means for holding one of said contacts in closed condition; actuating means for moving both of said contacts in the same axial direction from closed to open condition, said actuating means including first piston-cylinder means associated with said one contact and second piston-cylinder means associated with the other contact, said second piston-cylinder means including port means communicating with said chamber for continuously exerting a force on said other contact in said one axial direction whereby forcibly to engage said end portions of the contacts when same are closed and said latch means is holding said one contact; means for limiting axial movement of said other contact to a stroke less than that of said one contact; means for releasing said latch means to initiate opening movement of said contacts due to the force continuously exerted by said second piston-cylinder means; contact movement-responsive valve means for communicating said first piston-cylinder means with said chamber in response to movement of said contacts from closed condition to force said first contact in said one axial direction and move it beyond said other contact to its open condition when said other contact reaches the end of its stroke.
 5. In a gas-blast circuit-breaker as defined in claim 4 including a fixed switching contact bridging across said end portions of said pair of contacts and engaging said one contact when said pair of contacts are in closed condition, the stroke of said other contact being sufficient to carry it beyond said fixed contact whereby a circuit-breaking arc is preferentially drawn between said end portions of saiD pair of contacts.
 6. In a gas-blast circuit-breaker as defined in claim 5 including delayed action valve means for balancing the pressurized extinguishing gas across said first and second piston-cylinder means in delayed response to actuation of said means for releasing said latch means; and spring means for returning said pair of contacts to closed condition when said piston-cylinder means are so balanced.
 7. In a gas-blast circuit-breaker according to claim 6 wherein said delayed action valve means also causes return of said latch means.
 8. In a gas-blast circuit-breaker as defined in claim 4 including delayed action valve means for balancing the pressurized extinguishing gas across said first and second piston-cylinder means in delayed response to actuation of said means for releasing said latch means; and spring means for returning said pair of contacts to closed condition when said piston-cylinder means are so balanced.
 9. In a gas-blast circuit-breaker according to claim 8 wherein said delayed action valve means also causes return of said latch means. 